RSV is the leading cause of acute upper and lower respiratory tract infections (LRI) in adults, young children and infants. Serological evidence indicates that in the western world approximately 95% of all children have been infected with RSV by the age of two and 100% of children have been exposed by the time they reach adulthood (see Black, C. P., 2003, Resp. Care 48:209-31 for a review of the biology and management of RSV). In most cases the RSV infections will only cause minor upper respiratory illness with symptoms resembling that of the common cold. However, severe infection with the virus may result in bronchiolitis or pneumonia which may result in hospitalization or death. In a given year, around 91,000 infants are hospitalized with RSV infections in the United States. Infants who have been born prematurely or have a pre-existing lung disease are a high risk of severe infection and complications. These infections are responsible for 40 to 50% of hospitalizations for paediatric bronchiolitis and 25% of hospitalizations for paediatric pneumonia. Since the immune response to RSV infection is not protective, RSV infections reoccur throughout adulthood. In adults and older children, RSV infection has been associated with upper respiratory infection, tracheobronchitis, and otitis media. However, RSV in the institutionalized elderly can be more serious and is characterized by severe pneumonia and mortality rates of up to 20 and 78%, respectively. Adults with a previous history of heart or lung conditions are at a high risk for RSV infection.
RSV is a member of the order Mononegavirales, which consists of the non-segmented negative strand RNA viruses in the Families Paramyxoviridae, Rhabdoviridae and Filoviridae. RSV of humans (often also termed RSV or HRSV) is a member of the Pneumovirus genus of the sub-family Pneumovirinae within the Family Paramyxoviridae. Based on genetic and antigenic variations in the structural proteins, RSV is classified into two subgroups, A and B (Mufson, M. et al., J. Gen. Virol. 66:2111-2124). Other members of the Pneumovirus genus include viruses such as bovine RSV (BRSV), ovine RSV (ORSV) and pneumonia virus of mice (PVM) amongst others.
In addition to the genome features described above, family characteristics include a lipid envelope containing one or more glycoprotein species considered to be associated with attachment and entry of the host cell. Entry is considered to require a process by which the viral envelope fuses with the membrane of the host cell. Fusion of infected cells with, for example, their neighbours, can also result in the formation of fused multinucleate cells known as syncytial in some cases. The fusion process is believed to be glycoprotein mediated and is a feature shared with diverse enveloped viruses in other taxonomic groups. In the case of the Paramyxoviridae viruses of all genera characteristically express a fusion glycoprotein (F) which mediates membrane fusion.
While a RSV licensed vaccine is not yet available, some success has been achieved in the area of prevention for infants at high risk of serious lower respiratory tract disease caused by RSV, as well as a reduction of LRI. In particular, there are two immunoglobulin-based therapies approved to protect high-risk infants from serious LRI: RSV-IGIV (RSV-immunoglobulin intravenous, also known as RespiGam™) and palivizumab) (SYNAGIS®). RSV-IGIV (RespiGam, Massachusetts Public Health Biological Laboratories and MedImmune Inc, Gaithersburg, Md.) was licensed by the Food and Drug Administration in January 1996 for prevention of severe RSV lower respiratory tract disease in infants and children younger than 24 months with chronic lung disease (CLD) or a history of preterm birth (≦35 weeks' gestation). In June 1998, the Food and Drug Administration licensed Palivizumab (MedImmune, Gaithersburg, Md.) for administration as a monthly intramuscular injection for the prevention of serious respiratory disease caused by RSV in infants and children with a history of preterm birth (≦35 weeks' gestation) or CLD.
The only drug currently approved for the treatment of severe RSV is the antiviral medication, Virazole, also known as Ribavirin currently licensed for therapy of RSV pneumonia and bronchiolitis (Hall et al, 1983, N. Engl. J. Med., 308: 1443; Hall et al., 1985, JAMA, 254:3047. This agent has a broad spectrum antiviral with virustatic effects, and acts by inhibiting RSV replication. Unfortunately, the agent is toxic so that administration of the agent is confined to a hospital setting (Black, C. P., 2003, Resp. Care 48(3):209-31). Its administration is further complicated by the need to follow a strict procedural process when administering the agent in order to minimise the likelihood of certain adverse affects. The agent has a number of adverse effects including sudden deterioration of respiratory function (bronchiospasm). The efficacy of Virazole has remained controversial.
Accordingly, there remains an ongoing need for new compounds that are useful in the treatment of RSV infections.